1. Field of the Invention
The present invention relates to an optical recording medium utilizing a phase change by laser light irradiation, and more particularly to an optical recording medium adapted to higher recording densities.
2. Description of Related Art
Many devices are currently used as recording media for recording pieces of information. Among them, phase change type recording media are known, such as main recording media, which support the practical use of DVD""s (Digital Versatile Discs) which are recording media of voice, image and information.
Such a phase change type recording medium is formed with a recording layer including chalcogens as main components, in which the recording layer is locally irradiated by laser light so as to cause a phase change between a crystal phase and a noncrystal phase, to thereby conduct recording by utilizing the difference between optical characteristics in the respective phase states.
FIG. 4 shows a film constitution of a conventional phase change type-recording medium. In FIG. 4, the phase change type recording medium comprises a light reflecting side substrate 11 sequentially laminated thereon with: a reflecting film layer 12, a second dielectric layer 13, a phase-change recording layer 14 and a first dielectric layer 15, all of which layers are formed by a film-forming method such as a resistive heating vacuum deposition method, an electron beam vacuum deposition method, or a sputtering method; and a light incident side substrate 16 provided thereon by adhesion or by coating and curing.
The light reflecting side substrate 11 and light incident side substrate 16 are generally transparent in the visible light range, and it is possible to adopt therefor those made of: glass; a plastic resin such as polycarbonate; or an ultraviolet curable resin. Typically, the light reflecting side substrate 11 is provided thereon with a tracking-aimed guide groove serving as a rail for precisely guiding the travel of the light beam.
The reflecting film layer 12 reflects the laser light transmitted through the phase-change recording layer 14 so as to cause the thus reflected laser light to interfere with the laser light reflected by an upper surface of the phase-change recording layer 14, and adopted for the reflecting film layer 12 is a single metal material having a higher reflectivity such as Al, Au, Ag, Cu, Cr, an alloy including a plurality of kinds of such metals, and a mixture thereof.
The phase-change recording layer 14 is a material having a reflectivity to be changed by a phase change reversibly caused by laser light irradiation, and there is concretely adopted therefor an alloy mainly including Te such as Sbxe2x80x94Te, Gexe2x80x94Sbxe2x80x94Te, Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te, and Gexe2x80x94Inxe2x80x94Sbxe2x80x94Te. Optical recording media including phase-change recording layers made of such Te alloys have a higher crystallization speed and thus a shorter erasure time, thereby enabling a high-speed overwrite based on a single circular beam by simply modulating an irradiation power of laser light. The state of the phase-change recording layer just after film formation is an amorphous or noncrystal phase state. Thus, there is conducted an initialization treatment for bringing the whole of phase-change recording layer into a crystal phase, so as to form a recorded portion upon recording a piece of information into the phase-change recording layer. The recording is achieved by forming an amorphous phase portion within a crystallized state.
The first dielectric layer 15 and second dielectric layer 13 are arranged on both sides of the phase-change recording layer 14, respectively, so as to have: a protective function for avoiding a change of optical characteristics of the phase-change recording layer 14 due to a chemical change thereof such as oxidation; and an optical adjusting function for adjusting the reflectivities of recorded portion and erased portions in the phase-change recording layer 14 by film thicknesses, refractive indexes and optical absorptivities of these dielectric layers, respectively. Adopted as these dielectric layers are those materials having an excellent adhesive property to the phase-change recording layer 14 and reflecting film layer 12, and durability causing no cracks even due to long-term storage. Particularly, there have been conventionally adopted mixtures of ZnS and SiO2, since they have smaller film stresses and excellent adhesive properties to adjoining layers.
In the aforementioned conventional phase change type recording medium having the film constitution shown in FIG. 4, rewriting for a great number of times result in diffusion of constituent atoms of the first dielectric layer 15 and second dielectric layer 13 into the phase-change recording layer 14 to thereby change the composition of the phase-change recording layer 14, thereby causing a possibility of an occurrence of fluctuation such as of recording characteristic and erasure characteristic due to repetitively conducted rewriting.
To avoid such a deterioration of characteristics due to repetitive rewriting, there has been proposed a phase change type-recording medium having anti-diffusion layers (see JP-A-10-275360 [275360/1998], and JP-A-11-115315 [115315/1999], for example).
The phase change type recording medium having such anti-diffusion layers includes the anti-diffusion layers made of silicon oxide, aluminum nitride, germanium nitride, between the phase-change recording layer 14 and first dielectric layer 15 and between the phase-change recording layer 14 and second dielectric layer 13, respectively, in a manner to interpose the phase-change recording layer 14 therebetween, to thereby avoid: mutual diffusion of constituent atoms of the first dielectric layer 15 and second dielectric layer 13 and the phase-change recording layer 14; and a timewise change of the recording layer composition.
Meanwhile, JP-A-10-326434 (326434/1998) discloses a thick-film based phase change type recording medium provided with a high hardness layer having a sufficiently large film thickness and a hardness higher than that of the first dielectric layer 15, in a manner to be contacted with the laser light incident side of the first dielectric layer 15, so as to increase the mechanical strength of the medium and to reduce the deterioration at a write starting portion and a write ending portion of each of sectors in repetitive rewriting.
Recently, requirements for higher recording densities in optical recording media have become increasingly severe, and beam diameters of laser light are also promoted to be diminished for higher recording densities. One way to achieve the above includes shortened wavelengths of laser light.
Namely, in focusing a laser beam by an optical lens, the minimum beam diameter depends on the wavelength of the laser, so that the shorter wavelength allows further diminishing of the beam diameter. This means that recording densities of optical recording media can be increased inversely proportionally to the laser wavelength, so that there is a trend to replace light sources from currently used laser beams in the red color range to those in the violet range having wavelengths near 400 nm.
Meantime, in the aforementioned conventional phase change type recording medium, the laser light having a high energy density is locally irradiated onto the recording layer upon recording, thereby causing considerable mechanical strains in the recording layer. As such, repetitive rewriting for a great number of times result in repeated melting and solidification of the recording layer thereby easily causing a flow of the melted recording layer, to thereby cause problems such as deterioration of jitter characteristics, decreasing of amplitudes of reproduced signals, and a due number of rewriting operations is not sufficiently ensured.
Particularly, in case of adopting mark edge recording, the recording layer tends to flow due to recording and erasing more easily than pit position recording, thereby leading to more considerable strains at edge portions of record marks.
The cause of the above can be considered to be the facts: that those elements or compounds constituting the recording layer are segregated into shapes of record marks resulting from overheating by irradiation of laser light having a high energy density; and that the recording layer is brought to high temperatures exceeding the melting point of the recording layer, resulting in that the first dielectric layer and second dielectric layer contacting with the recording layer are thermally expanded to thereby deteriorate the mechanical strength of the medium such that these dielectric layers are bent toward the recording layer, thereby pushing out the melted recording layer to those locations at lower temperatures and in the recording track direction, thereby leading to an decreased amount of the recording layer material in the recording region.
Further, such a film thickness change of the recording layer and the physical deformation of the first dielectric layer, second dielectric layer and substrates resulting from thermal expansion problematically deteriorate the stability of tracking.
Meanwhile, in the aforementioned phase change type recording medium having the anti-diffusion layers, the thermal conductivity of the anti-diffusion layers such as silicon oxide, aluminum nitride or germanium nitride is larger than that of the first dielectric layer and second dielectric layer of the medium. This causes a problem such that the heat conduction characteristic is largely changed from the recording layer to the adjoining layers, i.e., that the heat at the recording layer due to the laser light irradiation tends to escape through the anti-diffusion layers to thereby excessively rapidly lower the temperature of the recording layer heated by the laser light irradiation, such that record marks are not correctly formed within the recording region upon recording, thereby deteriorating the jitter characteristics.
Moreover, in case of forming high hardness layers having higher hardness and larger thickness upon manufacturing optical recording media, the film forming speed is typically low, so that films having larger thicknesses problematically require a longer period of time for film formation, to thereby deteriorate the productivity.
It is therefore a main object of the present invention to provide an optical recording medium which sufficiently maintains a mechanical strength of the medium when adapted to higher recording densities, to thereby avoid mechanical strains of the recording layer due to heat even in case of conducting repetitive recording, and which medium particularly has a superior stability of record marks and an excellent resistance to repetitive recording in mark edge recording.
To achieve the object, the present invention provides an optical recording medium including a recording layer and a substrate, in which light is irradiated to the recording layer to conduct a phase change of the recording layer between an amorphous phase and a crystal phase to thereby record/erase information into/from the recording layer thereby enabling to rewrite the information through the change of optical characteristics by the phase change, wherein the optical recording medium further comprises at least a hardness enhancing layer and a first dielectric layer, such that the hardness enhancing layer, the first dielectric layer and the recording layer are laminated on the substrate in this order; wherein the hardness enhancing layer has a hardness higher than that of the first dielectric layer; and wherein the hardness enhancing layer has a film thickness Th having a value settled correspondingly to a film thickness T1 of the first dielectric layer, such that the film thickness Th and the film thickness T1 have a relationship of 0.4 less than Th/T1 and the film thickness Th is less than 15 nm.
In this way, according to the present invention, it becomes possible to sufficiently reinforce the entire medium to thereby realize enhancement of the mechanical strength of the medium, by providing the hardness enhancing layer 7 having a hardness greater than that of the first dielectric layer 5 and by appropriately selecting the substance constituting the hardness enhancing layer 7, as shown in FIG. 1. Particularly, even when laser beams having higher energies at shorter wavelengths correspondingly to higher recording densities are irradiated to thereby bring the phase-change recording layer 4 to high temperatures, the otherwise considerable mechanical strains of the phase-change recording layer 4 resulting from heat are avoided by virtue of the hardness enhancing layer 7, thereby allowing to avoid the flow of the phase-change recording layer 4 and the deformation of the whole of the phase-change recording layer 4 to be otherwise caused thereafter.
Particularly, it becomes possible to stabilize record marks in mark edge recording to thereby obtain reproduced signals faithful to recorded signals, and to ensure a due number of rewriting times.
Further, the hardness enhancing layer 7 is laminated without contacting the phase-change recording layer 4, so that the heat conduction characteristic of the phase-change recording layer 4 is not changed, i.e., the thermal conductivity of the layer adjoining to the phase-change recording layer 4 is not changed, thereby allowing to obtain a stable recording sensitivity.
Meantime, if the film thickness Th of the hardness enhancing layer 7 and the film thickness T1 of the first dielectric layer 5 have a relationship of Th/T1xe2x89xa60.4 therebetween, the film thickness Th of the hardness enhancing layer 7 becomes small relative to the film thickness T1 of the first dielectric layer 5, to thereby restrict the effects of the hardness enhancing layer 7 for enhancing the mechanical strength of the medium and for avoiding the mechanical strains of the phase-change recording layer 4 resulting from heat. As such, when the phase-change recording layer 4 is brought to high temperatures by irradiation of laser light having a high energy density upon recording, the phase-change recording layer 4 is caused to flow, to thereby cause a deformation of the whole of phase-change recording layer 4, thereby deteriorating the stability of record marks and the stability of tracking.
It is thus desirable that the film thickness Th of the hardness enhancing layer 7 and the film thickness T1 of the first dielectric layer 5 has the relationship of 0.4 less than Th/T1, such that the film thicknesses of the hardness enhancing layer 7 and first dielectric layer 5 may be thinly formed insofar as satisfying the film thickness ratio. This allows the mechanical strains otherwise caused within the hardness enhancing layer while maintaining the mechanical strength of the medium and avoiding mechanical strains of the phase-change recording layer 4 resulting from heat, to thereby avoid an occurrence of separation and fine cracks at interfaces between adjoining layers so that the resistance to repetitive recording is improved.
Moreover, the hardness enhancing layer 7 and first dielectric layer 5 can be deposited in thinner film thicknesses, to shorten the film-forming time upon manufacturing, thereby improving the productivity.
When the film thickness Th of the hardness enhancing layer 7 is less than 15 nm, jitter characteristics become excellent.
In the present invention, it is desirable that the film thickness of the hardness enhancing layer is in a range of 1 nm to 10 nm.
In this way, according to the present invention, even when the film thickness of the hardness enhancing layer 7 is on the order of 1 nm to 10 nm, the mechanical strength of the medium can be sufficiently maintained to thereby avoid mechanical strains of the phase-change recording layer 4 resulting from heat. Nonetheless, defining the film thickness of the hardness enhancing layer 7 in the aforementioned manner allows the film thickness of the first dielectric layer 5 to be freely set insofar as satisfying the conditions of 0.4 less than Th/T1 such that the first dielectric layer 5 is allowed to have a required dimension, to thereby optimize the optical recording medium such as correspondingly to usage and required performances upon designing the optical recording medium.
Further, thinly forming the film having the higher hardness avoids an occurrence of larger stresses within the film, to thereby avoid an occurrence of separation and fine cracks at interfaces between adjoining layers. Jitter characteristics are particularly excellent, within such a range where the film thickness of the hardness enhancing layer 7 is small.
In the present invention, it is desirable that the film thickness of the first dielectric layer is in a range of 1 nm to 5 nm.
In this way, according to the present invention, even when the film thickness of the first dielectric layer 5 is on the order of 1 nm to 5 nm, the protective function and optical adjusting function of the first dielectric layer 5 for the phase-change recording layer 4 is ensured so that the recording/reproducing characteristics are never deteriorated. Further, thinly forming the film thickness of first dielectric layer 5 allows to promote that effect to be achieved by the hardness enhancing layer 7, which effect restricts the otherwise thermally caused flow of the phase-change recording layer 4 accompanying to the higher temperature of the phase-change recording layer 4 due to irradiation of laser light having a high energy density.
Moreover, the film thicknesses of the layers are formed to be sufficiently thin upon manufacturing the optical recording medium, so that the film-forming time is shortened, thereby leading to an improved productivity.
In the present invention, it is desirable that the hardness enhancing layer has a Vickers hardness of 4,000 N/mm2 or higher, or the hardness enhancing layer has a Knoop hardness of 4,000 N/mm2 or higher.
In this way, according to the present invention, the mechanical strength of the medium can be enhanced even when the hardness enhancing layer 7 is thin if the Vickers hardness of the hardness enhancing layer 7 is 4,000 N/mm2 or higher from the viewpoint of the mechanical strength of the medium and the viewpoint of the Vickers hardness of the first dielectric layer 5, thereby allowing to restrict the flow of phase-change recording layer 4 and to ensure a due number of rewriting times. The material of such a hardness enhancing layer 7 includes a metallic compound such as a metal nitride, metal oxide, metal carbide, metal sulfide, metal selenide, and a mixture thereof.